This invention relates to the production of methyl tertbutyl ether (M T B E) by reacting methanol with the isobutene contained in a C.sub.4 cut.
Such a reaction is known as being balanced and accordingly, it is difficult to obtain high conversion rates. It is however possible to obtain them by making use of relatively complex techniques which considerably increase the cost of the operation. In most cases, only a relatively low isobutene proportion of the C.sub.4 cut is converted.
However, this is not acceptable for certain utilizations of the n-butenes of the residual C.sub.4 cut, such for example as the synthesis of maleic anhydride or the production of 1-butene polymers or copolymers.
The invention has for object to obtain a high conversion rate of the isobutene, resulting in the obtainment of a C.sub.4 cut free to a large extent from isobutene, which can be used particularly for the specific above-mentioned applications.
The invention will be better understood by comparison with prior art processes, and with reference to FIGS. 1 and 2. When making use of a single reactor (1) or of an assembly including several reactors, it has been possible to obtain, by mere passage over the catalyst, isobutene conversions of from 93 to 98% with molar ratios methanol/isobutene at the reactor inlet (1) sufficiently low (for example from 1.02 to 2, preferably from 1.05 to 1.3) for avoiding the requirement, for recovering methanol, of a distillation column in addition to column 3 operated under high pressure, for example 10 to 30 bars; substantially all the methanol passes at the top and it is accordingly useless to make use of an additional distillation column for fractionating the bottom effluent from column 3 to a methanol-MTBE azeotrope, at the top, and a purified MTBE stream, at the bottom. In order to increase the isobutene conversion, the process described in French Patent Specification No. 79 03937 of Feb. 14, 1979 recycles the distillate from column 3 to the inlet of reactor (1) through line 10, shown in dashed line in the drawings. It is thus possible to obtain conversion rates of about 98-99%. If still higher conversion rates are desired without additional reactors, the molar ratio methanol/isobutene must be much higher. Particularly by mere passage of the charge over the catalyst, it is required that the molar ratio CH.sub.3 OH/isobutene be about from 9 to 10 in order to obtain a conversion rate of 99.8%. This results in serious disadvantages due to the fact that the effluent from the reactor then contains substantial amounts of methanol which have to be separated from the MTBE and from the C.sub.4 cut. Such a separation is difficult and expensive, irrespective of the operating mode.